In the field of wireless digital communications, several techniques are known for ensuring the division of a given frequency spectrum between several users, such as the TDMA (or Time division multiple access) technique, FDMA (frequency division multiple access) and CDMA (code division multiple access), the latter being based on the use of the Walch-Hadamard orthogonal codes.
The CDMA technique is used in 3rd generation telephony, in particular with the development of UTMS (Universal Mobile Systems Telecommunication) standard, as defined by the 3GPP (3rd Generation Partnership Project) organism of standardization.
As it is known, in wireless communication, a signal is often the object of dispersions, reflections, fading etc. . . . , causing the reception, within the receiver, of a multiplicity of shifted signals one against the others, characteristics of many paths.
One treats these reflections, multiple dispersions of a signal by means of a rake receiver, which comprises a multiplicity of units (fingers) for the treatment of the various reflections, shifted one against the others, in order to allow, after treatment, the summation of all the elementary contributions of the reflections resulting from multiple paths.
FIG. 1 illustrates the situation of a user equipment 2 comprising a rake receiver 3 likely to treat a number N of distinct reflection paths from a same signal emitted by a basic station 1. As a matter of clarity, only three paths, respectively 11-13, are represented in the figure and correspond to three distinct contributions of a same signal arriving to the receiver at shifted moments, and with different amplitudes.
Generally, the assignment of the fingers of a rake receiver is carried out by means of a pilot detection mechanism (“Common Pilot Channel” (CPICH)) and by its possible reflections. For this purpose, one achieves a measurement of the energy of the received signal and a comparison with a threshold value, as that illustrated in FIG. 2, in order to detect the different reflection paths. FIG. 2 shows that, following a first significant path of high amplitude corresponding to direct signal reception (Line of Sight), two reflection paths of lesser energy follow. The mechanism of threshold detection is regulated in a manner to avoid the false detections (Constant False Alarms Rate (CFAR)) but can cause an omission of one or more paths presenting a lesser energy, as the case of the 2nd reflection in FIG. 2.
In order to ensure a maximum effectiveness at the receiver, it is important that the process of assignment and deallocation of the correlation units (Finger) of the rake receiver is particularly reactive. Indeed, because of the receiver mobility but also because of the changing characteristics of the communication channel, the propagation paths offer multiple reflections that change quickly and it is essential that the rake receiver can follow these changes rapidly and precisely.
Obviously, this precision and this reactivity in the process of assignment and deallocation of the correlators (Finger) of the rake receiver initially determine the level of BLER (Block Error Rate).
In a second level, the reactivity of the receiver determines the effectiveness of the process known as HANDOFF, allowing one mobile equipment, in communication, to switch from a first to a second base station. Indeed, one recalls that in the UMTS context, the receivers must permanently follow the reception of data emanating from several base stations in order to allow, in a cell limit, the switching (handoff) from one station to another one without disconnection of the communication. For this reason, it is essential that a rake receiver is able to follow, precisely and rapidly, not only the propagation paths of its own base station, but also those emanating from the neighboring base stations.
It can thus be observed how critical is the problem of assignment of the resources of the rake receiver, and particularly that of each correlation unit which composes it.
One tries to avoid as much as possible the assignment of a correlator to a propagation path which would be proved, later on, not to correspond to a true reflection of the transmitted signal.
Conversely, an erroneous detection of a propagation path must be restored as soon as possible in order to deallocate the unduly affected correlator with the false path and to allow a new assignment.
Generally, in order to solve this critical problem, the assignment of a correlator (Finger) to a propagation path is based on one or more indicators, such as, for example, the power of the signal or the signal to noise ratio measured on the frame level.
In order to avoid false detections, likely to generate false assignments, it is necessary, in these known techniques, to carry out several consecutive measurements on this or these indicators and to integrate the result of these measurements on several frames.
This results, and that is a major disadvantage, to a high time constant in the assignment process and, finally, to a low reactivity of the receiver.
It is advisable to be able to gain from an effective and especially from a faster process in the assignment of the correlator of a rake receiver and the deallocation of the resources of this same receiver, deallocation necessary during a false detection or at the time of disappearance of a given propagation path.